Wood Framed Shear Walls in Single Family Residence 1

[RockEngineer]

I am trying to get some other practicle opinions on how working engineers actually handle the lateral design of wood framed single family dwellings in seismic zone 2 or 3 areas. I have read several articles and papers on the controversy of designing walls for flexible diaphragm (shear load distributed based on wall length) and rigid design (shear load distributed based on stiffness). There does not seem to be a clear consensus and the whole house test data and historical data don't clear up the matter. I know there are programs like Woodworks and Latpro that will calculate the distribution of shear walls but they are only as good as the assumptions you give them. Is the load distribution diaphragm flexible or rigid or somewhere inbetween? How do interior gypbd shear walls affect the distribution? If your building is over 34 feet between exterior walls you have to use interior gypboard shear walls and even if you don't count them those interior walls will affect the shear distribution. How much does the quality and care of the builder affect the strength. Building departments sometimes require a table from the engineer showing the distribution of shear forces to every shear wall throughout the building. There are a lot of assumptions go into how the shear is distributed. In this forum I see a lot of recommendations for using rigid diaphragm design and stiffness and then comments that they prefer to do this by hand rather than with a program like latpro or woodworks. The true lateral stiffness of each panel depends a lot on the location of each holddown. Do engineers detail out every shear wall with all the hold downs and sheathing orientation. This is the reccomendation of some groups such as the Curee Project at

http://www.curee.org/projects/woodframe_project/element3/091001B-I-final.pdf

What are real working engineers who deal with engineering single family residences doing? Does anyone really lay out all the sheathing for all the roof, floor and wall diaphragms on their drawings for single family residences. On your drawings do you show all walls without window and door openings as shear walls or do you designate the minimum number of walls as shear walls and show that they are below the acceptable values for the nailing pattern you required and know that the other walls you haven't designated will lower the shear and make your calculations conservative?

After 22 years engineering in many areas I still have lots of questions and very few answers when it comes to standard framed construction. Anyone out there have the answers to these simple/complex questions.

 

[ishvaaag]

I can only give you a comment from the distance, since we (in Spain) don't practice at all such kind of construction.

According to the general good practices of the science of construction as I understand them now, any lateral strength coming from purportedly non-structural panels would be outrightly dismissed for all design purposes. The lateral resisting system sets a warranted and tolerable level of lateral deformation for everything else. The contrary would be in my view an alternative adopted with some special target in mind, usually solving some technical or economical problem.

Then, if you ensure firm tie-down of the lateral resisting panels, one should assign the solicitations as their stiffness at their place demands. This is not so difficult once one owns 3D design programs. Other thing is that by code or other aceptable engineering source one may be able to use simplified ways to assign forces for the individual design of the shearwalls, but this is to be in my opinion to be thought worse than what above.

Respect details, here it is expected that the drawings contain anything of structural meaning. However it is obvious that (out of this field) there remains work for steel fabricators, precast providers and even reinforcement detail IF someone is asking more detail than usually provided. This also uses to go a lot with the kind of work, premium jobs (of higher fees) are expected to hold more detail, sometimes quite exhaustive. In the end I think that if the drawings are meant to be used in a limited areaa where there is definite custom of having or not such and such detail, that might turn acceptable practice as long as the code does not demand otherwise. Yet, out of professional responsability, one must always express anything it thinks required for the satisfactory construction of his design.

 

[ERV]

Right, this is a simple yet complex question, which can have many answers.

In seismic zones 2 & 3 wind usually governs lateral force requirements in wood frame construction (unless the building is unusually heavy).

We usually try to keep the aspect ratio of the building (width to depth) to 1.5 to 1 by cutting the building into a series of boxes. A building 26' x 78' would have an interior shear wall around 39', or as close as possible. Never mind all those gyp board shear walls (they do not have much value).

Walls that do not have any openings are a bonus. Walls that have openings have to be calculated according to the piers and can be cumulative for the length of the wall, adequate holdown against uplift must be provided at piers.

In California in seismic zone 4 we usually list every shear wall, drag strut, and reference to details on a seperate sheets devoted just for that purpose (shear plan). On a single story residence the roof plan contains diaphragm elements, such as drag trusses and reference to details. On a two story residence the floor diaphragm elements are listed on the second floor framing plan with reference to details.Every connection must be detailed (sometimes 100's). All shear walls, diaphragms, hold down requirements, fastener schedules, etc. are required to be calculated in the engineering report.

I do not like to use or specify gypsum board shearwalls, they just don't do what they are intended for (over rated). Thermo-ply Structural Grade Red .113 is a good choice under Gyp board.

Anyway I could go on forever.

The link above is an experimental project, and the results (they hope) to be used in future building codes, or at least that is what I got out of it.

 

[ERV]

A plywood diaphragm acts as a horizontal deep beam to collect and transfer lateral forces to the shear walls. Structural wood panels form the beam web to resist shear force, purlins act as web stiffeners, and the boundary members normal to the load form the flanges to resist flexural effects. Shear stresses are assumed uniformly distributed across the depth of the diaphragm. The boundary members, acting as the flange or chord of the diaphragm, may consist of the double top plate of a wood framed shear wall. The contribution of the plywood sheathing to the flexural capacity of the deep beam is neglected and the chords are assumed to resist the total applied moment by developing axial forces that provide a couple equal and opposite to the moment.

The determination of diaphragm flexibility is dependent on the relative deformation of the diaphragms and shear walls in a structure. A building with wood framed diaphragms may not necessarily be considered a flexible structure as this determination depends on the stiffness of the shear walls. A building with a wood framed roof and concrete or masonry shear walls will behave as a flexible structure because the walls are highly rigid. A structure with a wood framed roof and wood shear walls may not necessarily be considered flexible as this depends on the relative deformations of the diaphragms and shear walls.

A diaphragm is considered flexible, in accordance with IBC Section 1602.1, when the midpoint displacement of the diaphragm, under lateral load, exceeds twice the average story drift.

In accordance with IBC Section 2302.1, a diaphragm is considered rigid when the midpoint displacement of the diaphragm does not exceed twice the average story drift. The diaphragm is sufficiently stiff to distribute torsional moments and allowance must then be made for additional forces created by torsional effects with the diaphragm and supports to undergo rigid body rotation. The distribution of loading to the supports is proportional to their relative stiffness and is independent of the tributary areas supported.

Since the design of a structure is affected by its classification as either a rigid or flexible structure, a determination of the classification is necessary at the commencement of design. Small light frame buildings have traditionally been designed using flexible diaphragm assumptions and it has been proposed that one and two-family dwellings continue to use this approach. SEAOC Section C805 and NEHRP Section 5.4.4.1, however, recommend and envelope approach for less conventional structures with the most severe effects from both flexible and rigid analysis adopted for design.

A collector is defined in IBC Section 2302.1 as a diaphragm element, in line with the applied force, that collects and transfers diaphragm shear forces to the vertical shear walls. Where shear walls are discontinuous or re-entrant corner irregularities are present in a building, collector elements or drag struts are required to ensure deformation compatibility and prevent localized tearing of the diaphragm. The drag strut transfers the shear originating in the unsupported portion of the diaphragm to the shear wall.

A shear wall is defined in IBC Section 2302.1 as a wall designed to resist lateral forces parallel to the plane of the wall. A shear wall acts as a cantilevered diaphragm in transferring lateral forces from a horizontal diaphragm to the foundation.

Because of the light weight of timber framed construction, it is usually necessary to provide holdowns at the ends of plywood sheathed Overturning restraint is determined using service level load combinations.
Holdowns do not necessarily add to the stiffness of a shear wall.

 

[jeat]

Try "Timber Solutions Manual " Author;David W. Duquette P.E. Publisher Da Vinci Publishing, New York, New York. 1997 ISBN: 0-9656181-0-2 cost 49.95 Dollars US

 

[RockEngineer]

Thanks guys. I still have a hard time believing that all the practicing engineers out there when they have a single family residence that is not prescriptive do a deflection analysis of the roof and floor diaphrams to determine if they need to consider the structure as rigid or flexible for shear wall load distribution and then detail out every floor and roof diaphragm and shear wall. Maybe I'm just lazy or I don't charge near enough.